Attackers are using two known exploits to silently install ransomware on older Android devices when their owners browse to websites that load malicious advertisements.

Web-based attacks that exploit vulnerabilities in browsers or their plug-ins to install malware are common on Windows computers, but not on Android, where the application security model is stronger.

But researchers from Blue Coat Systems detected the new Android drive-by download attack recently when one of their test devices -- a Samsung tablet running CyanogenMod 10.1 based on Android 4.2.2 -- became infected with ransomware after visiting a Web page that displayed a malicious ad.

"This is the first time, to my knowledge, an exploit kit has been able to successfully install malicious apps on a mobile device without any user interaction on the part of the victim," Andrew Brandt, director of threat research at Blue Coat, said in a blog post Monday. "During the attack, the device did not display the normal 'application permissions' dialog box that typically precedes installation of an Android application."

Further analysis, with help from researchers at Zimperium, revealed that the ad contained JavaScript code that exploited a known vulnerability in libxslt. This libxslt exploit was among the files leaked last year from surveillance software maker Hacking Team.

If successful, the exploit drops an ELF executable named module.so on the device that in turn exploits another vulnerability to gain root access -- the highest privilege on the system. The root exploit used by module.so is known as Towelroot and was published in 2014.

After the device is compromised, Towelroot downloads and silently installs an APK (Android Application Package) file that's actually a ransomware program called Dogspectus or Cyber.Police.

This application does not encrypt user files, like other ransomware programs do these days. Instead, it displays a fake warning, allegedly from law enforcement agencies, saying illegal activity was detected on the device, and the owner needs to pay a fine.

The application blocks victims from doing anything else on the device until they pay up or perform a factory reset. The second option will wipe all files from the device, so its best to connect the device to a computer and save them first.

"The commoditized implementation of the Hacking Team and Towelroot exploits to install malware onto Android mobile devices using an automated exploit kit has some serious consequences," Brandt said. "The most important of these is that older devices, which have not been updated (nor are likely to be updated) with the latest version of Android, may remain susceptible to this type of attack in perpetuity."

Exploits like Towelroot are not implicitly malicious. Some users willingly use them to root their devices in order to remove security restrictions and unlock functionality that is not normally available.

However, because malware creators can use such exploits for malicious purposes, Google views rooting apps as potentially harmful and blocks their installation through an Android feature called Verify Apps. Users should turn on this feature under Settings > Google > Security > Scan device for security threats.

Upgrading a device to the latest Android version is always recommended because newer versions of the OS include vulnerability patches and other security improvements. When a device goes out of support and doesn't receive updates anymore, users should limit their Web browsing activities on it.

On older devices, they should install a browser like Chrome instead of using the default Android Browser.

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